A comparative study of the development of the fetal testis and ovary in the monkey (Macaca fascicularis).

The gonadal development of the Macaca fascicularis fetus was studied between 37 and 118 days on serial semi-thin and thin sections. The testis and the ovary began to differentiate at the same age (37 days); the definitive architecture of the testis was acquired at 43 days, while a cortex and a medulla did not form in the ovary until 55 to 60 days. In spite of the time-lag and the divergent development, the testis and the ovary evidenced three comparable stages; the main event of these stages was the centrifugal role of the mesonephros. The first stage (37-43 days) included the centrifugal and antero-posterior differentiation of the sex cord anlages from the mesonephric mesenchyme in contact with the proximal loops of the anterior tubules (for a detailed study see Dang and Fouquet, 1979). From 43 days (second stage), a remainder of the mesonephric mesenchymal blastema of the gonad supplied the rete system. The mesonephric tubules fused secondarily with that system which was connected to the sex cords. Whereas in the testis, the rete blastema did not play a direct role in organizing testicular structures, but only in forming excretory pathways, in the ovary, it invaded the medulla (whose initial sex cords degenerated) and penetrated to the ovigerous cords of the cortex. The rete ovarii blastema was probably the major source of periovocyte cells. The third stage included the differentiation of a steroidogenic interstitial tissue (from 50 days in the testis; at about 60 days in the ovary) and is further involution; these processes were similar in both sexes. Observation of the fine structure showed the development of the male and female gonocytes to be the same; the prespermatogonia and the oogonia could be characterized by the formation of nuclear vacuoles. The Sertoli cells and the periovogonial cells showed the same features.